Requirements on transducers for ultra-high recording densities (greater than 60 Gb/in2) place certain constraints on the properties of the read and write heads needed to achieve this. These fundamental constraints have a profound influence on the design and fabrication of the read/write transducers. To achieve extremely high recording densities, Giant Magnetoresistance (GMR) reader design has to be capable of very high linear bit density (BPI) and also very high track density (TPI). Consequently, GMR devices continue to be pushed to narrower track widths and to thinner free layers to maintain high signal output in spite of reductions in track width and reduced gap length.
Ultra-thin free layers as well as MR ratios are very effective to obtain high signal output. Ultra-thin free layers having moments equivalent to 37 Å NiFe (20 Å CoFe) made of a composite CoFe—NiFe—Cu layer, is capped with 10—20 Å Ta or TaO. The large GMR ratio obtained from such a very thin free layer is due to: (a) the Cu HCL (high conductivity layer) which improves the mean free path of a spin-up electron and maintains the mean free path difference between spin-up and spin-down electrons, and, (b) the bottom spin valve structure which provides very good specular reflection at the Ru/CoFe and CoFe—NiFe—Cu(free layer)/Ta or TaO interfaces.
There are other features that make the bottom spin valve most suitable for extremely high recording densities. It is shown that the thin free layer of the bottom spin valve is magnetically softer than that of the top spin valve. GMR magnetostriction in the top spin valve increases asymptotically with the reduced free layer thickness. For the bottom spin valve, the magnetostriction can be attenuated by increasing the CoFe thickness in a CoFe—NiFe composite free layer. A thicker CoFe also improves the GMR ratio (Dr/r). One unique feature for the bottom spin valve is that the sensor longitudinal biasing can be made by a patterned exchange bias.
Spin valves with contiguous hard bias to achieve sensor stability are known to suffer amplitude loss due to the field originating from the hard bias structure. One approach that has been proposed to overcoming some of the amplitude loss and stability concerns has been to use a lead overlay design. In lead overlay design, MR sensor track width is defined by conductor lead edge while the contiguous hard bias junction is placed outside the conductor lead. The overlap length between the lead overlay and the hard bias junction should be less than 0.1 microns. This requirement imposes a great challenge to the photo-lithography.
An example of a bottom spin valve structure that is typical of the prior art is shown in cross-section in FIG. 1. Seen there is bottom magnetic shield 15 (commonly referred to as S1) which is coated with lower dielectric layer 17 (and commonly referred to as D1). Over this is seed layer 10 (typically nickel-chromium) which is, in turn, coated with pinning (antiferromagnetic) layer 14. Layer 13 is the pinned layer and layer 12 is the non-magnetic, electrically conductive layer. Layer 11a is the free layer, layer 11b being formed of the same material as layer 11a. Layer 19 is a capping layer while layer 114 is an antiferromagnetic layer that serves to provide longitudinal bias to the spin valve.
Continuing with FIG. 1, layer 110 is conductive material that serves for the formation of leads to the device. Layer 18 is the upper dielectric layer (commonly referred to as D3). Finally, layer 16 is the upper magnetic shield, which is commonly referred to as S2.
One key factor to improving BPI is to reduce the reader gap length (commonly called the shield-to-shield spacing). This is equal to the distance between S1 and S2 within the GMR sensor area in FIG. 1. For a 100 Gb/in2 recording density design, the sensor track width is 0.1 microns, and the gap length (S1/S2) is around 600 Å. For a 300 Å thick GMR stack, even for a very thin D1 (about 140 Å), D3 is less than 160 Å thick. The greatest concern with a very thin dielectric D1/D3 layer is (a) sensor to shield shorts and (b) dielectric breakdown.
A routine search of the prior art was performed with the following references of interest being found:
In U.S. Pat. No. 5,302,461, Anthony shows dielectric layers for MR heads including oxides of Ta, Hf, Zr, Y, Ti or Nb. The invention pertains to an MR (as opposed to a GMR) read head. The metal is deposited directly onto the MR plates and then allowed to oxidize.
Yamamoto et al. in U.S. Pat. No. 5,919,581 show a MR with a shield layer. Hsiao et al. in U.S. Pat. No. 5,999,379 and Kawano et al. in U.S. Pat. No. 5,432,734 disclose other MR structures with dielectric layers.